The invention describes an automatic control system for a hydrostatic transmission, especially as a propulsion system for vehicles, in which the control of the transmission ratio relates to the speed of the prime mover and the sum of loads on the prime mover is automatically matched to the power limit of the prime mover.
Hydrostatic transmissions for vehicles with a control system related to the speed of the prime mover (normally combustion engine) exist in various types. All existing systems are of similar principle, which is described as follows:
Displacement of the pump of the hydrostatic transmission (closed circuit type) is controlled by a hydrostatic cylinder which is actuated by a delta pressure signal. The delta pressure signal is generated using a flow across a fixed throttle. The flow itself is generated by a measuring pump which is running at the speed of the prime mover, or a certain proportion of that speed. The generated delta pressure signal to displace the pump is approximately proportional to the speed squared. The described system gives a certain relation between the speed of the prime mover and the ratio of the hydrostatic transmission.
The operator of the vehicle controls the speed of the prime mover with a pedal.
The described system doesn't provide a control of transmission ratio to cause the combustion engine to operate always in optimum condition, especially an optimum control is impossible, if there are auxiliary systems driven by the same prime mover as the propulsion system. Due to the insufficient control system, the prime mover shows severe variations in speed depending on load; in maximum speed drops to 50% of the rated speed and in extreme situations the engine stalls.
To get a better performance, systems are known, where the orifice of the speed measuring throttle is changed with the position of the prime mover control pedal. This feature changes the proportion between speed of the prime mover and transmission ratio, depending on the position of the control pedal for the prime mover. Other systems use, for instance, the position of an injection pump, in case of a Diesel engine, to influence hydrostatic transmission ratio. Other systems also use a centrifugal force governor to generate a control signal which influences the transmission ratio to reduce engine stall.
The described control systems are insufficient due to the requirements of optimum power control, optimum acceleration control (usage of full prime mover power), and optimum braking performance (defined deceleration).
The described controls become more complicated as they more nearly fulfill the requirements. For advantageous performance in the braking condition of a vehicle, well known systems use a so-called inch-valve (which is used for other purposes, too). Actuation of the inch-pedal reduces the delta pressure signal which controls pump displacement.